1. The Field of the Invention
The invention relates to the separation of oil and water due to gravity in a subterranean petroleum production well. In particular, the present invention concerns the production of the oil to surface, and the disposal of the water to a zone in the same wellbore, using a reciprocating sucker rod pumping system.
2. The Prior Art
Typically, oil wells produce a significant amount water. Water can often be as high as 99+% of total production. Traditionally, all water has been brought to the surface along with the oil. The oil and the water has been separated at surface by a variety of means and then disposed of in a variety of manners. This process has extensive costs associated with it. It takes big equipment and lots of power to lift to surface the large amounts of fluid required to retrieve a small proportion of oil. There are then costs associated with separating the oil from the water at surface, handling the then pure (i.e. no oil), but still possibly corrosive water, and disposing of it in a dedicated disposal well or by other means. The concept of "water control" to reduce operating costs and increase hydrocarbon production is receiving greater and greater attention in these competitive economic and environmentally conscious times.
One approach that has been taken to reduce these lifting and handling costs is to separate and dispose of the water downhole in the same wellbore. Most inventions facilitate the actual separation of the oil from water by a variety of mechanisms and apparatus. These include the use of filter systems, cyclones, and built in chambers where the oil is allowed to separate from the water by the force of gravity. U.S. Pat. No. 4,766,957 even describes under reaming a section of wellbore for water accumulation. Any actual apparatus that is required to facilitate oil/water segregation complicates the overall production system, and as a person familiar in the art will understand, should be avoided whenever possible.
Most downhole separation processes are driven by some form of surface or subsurface pump and/or include a control mechanism. Many of these separation and disposal processes are directed toward the deliquification of gas wells and/or toward flowing oil wells where there is enough reservoir energy to bring the desired hydrocarbons to surface such that no artificial lift system is necessary. However, a large proportion of the world's producing oil wells do not flow on their own and some form of artificial lift is required.
Most inventions are dedicated to disposing of water into a zone below the actual hydrocarbon producing zone. Unfortunately, many producing petroleum wells do not have an acceptable disposal zone below the production zone but do have one above the production zone. U.S. Pat. No. 5,579,838 specifically describes a method of disposing above the production zone. Again however, although not specifically stated, this process must be directed at gas wells and flowing oil wells as there is no provision for artificially lifting fluids to surface.
As already stated, most inventions facilitate the actual separation of the oil from water by a variety of mechanisms and apparatus. It is possible however to allow the force of gravity to segregate the oil from the water while it is still in the wellbore. The additional challenge however, is to artificially lift the desired hydrocarbons to surface, while disposing of the water to a zone in the same wellbore. Two inventions are noted.
UK Patent Application GB 2,248,462 describes the use of a progressive cavity pump (PCP) form of artificial lift. There are basically two PCP's connected together at the rotors. The rotor and stator combinations are configured in such a way that the upper set pumps oil up to surface and the lower set pumps water down to a disposal zone. Although incredibly simple and effective there are some inherent disadvantages to using a PCP in this application. Historically, PCP's have had specific limitations over other common forms of oil well artificial lift (i.e. reciprocating sucker rod pumps and electric submersible pumps). Firstly, because of the required use of an elastomer stator, the serviceability in a pure water application and/or in light oils containing aromatics is severely restricted. Secondly, PCP's have limited pressure capabilities which restrict their use in deeper wells and more specifically, in a separation/disposal application, restrict their use in wells where the disposal zone might have a high reservoir pressure or low infectivity. Thirdly, to get the fluid to warrant the inherently high service and repair costs, most PCP are still in a "tubing pump" configuration. That is, the entire pump needs to be run and retrieved on tubing, which to someone familiar in the art, will understand is a distinct disadvantage when compared to a reciprocating sucker rod pump, the embodiment of the current invention, which can be run and retrieved with the sucker rod string alone and does not require the "pulling" of both the rods and then the tubing. Finally, and most importantly with respect to wellbore gravity separation, a PCP is a constant flow pump and allows no "dead time" for additional gravity segregation of the oil and the water. A reciprocating rod pump on the other hand, only produces fluid on the upstroke. This means that half of the operating time (the downstroke) is "dead", allowing for even better gravity separation in the wellbore.
U.S. Pat. No. 5,497,832 describes a method of oil/water wellbore gravity separation and same wellbore water disposal using a reciprocation rod pump system. The Dual Action Pumping (DAP) system as described overcomes many of the limitations of the PCP pump above. However, the DAP does have some distinct disadvantages of its own. Firstly, the DAP is configured as a tubing pump, which means the whole system has to be run and retrieved on tubing as with the PCP. Secondly, the DAP as described in the patent requires the use of several conventional ball and seat valving systems that are attached external to the regular smooth profile of the "tubing pump". A person familiar in the art will understand that this will severely reduce the ruggedness of any downhole tool. This condition could easily result in actual physical damage when running or retrieving the system. Thirdly, and most importantly, the DAP injects the disposal water on the downstroke. Since there will always be a resistance to flow, due both to reservoir formation pressure and to the limited permeability of the formation, the DAP is required to create a downward force on the downstroke. This is not a typical condition for sucker rod pumping where all the load is taken on the upstroke. Again, a person familiar in the art will understand that, with an opposing upward force on the downstroke, the sucker rods in the well bore will tend to buckle, reducing bottom hole pump stroke and leading to a myriad of other potential mechanical problems in the production system. Although special sucker rod string design can effectively overcome small upward forces on the downstroke, the solutions will become impractical at higher disposal zone reservoir pressures and low permeabilities. Finally, the DAP patent does not suggest that the system can be utilized with an above production disposal zone.
As can be seen, although prior art has utilized wellbore gravity separation and linked it with common methods of artificial lift, there is still a need to overcome limitations in serviceability, ruggedness, rod loading, and disposal zone location.